The major objectives of the project are the elucidation of the mechanisms by which insulin, catecholamines, thyroid hormones, glucocorticoids and growth hormone regulate the metabolism of isolated rat fat cells. Emphasis is placed on the role of cyclic AMP in the regulation of lipid mobilization from adipose tissue. The four specific areas being investigated are: 1. Adenosine regulation of hormone-stimulated fat cell cyclic AMP metabolism and lipolysis is being examined with respect to adenosine binding to and adenylate cyclase activity of purified fat cell plasma membranes. We are attempting to extract, isolate, purify and characterize the plasma membrane component which binds adenosine. 2. The influence of thyroid hormones on the regulation of catecholamine action in fat cells is being examined. The impaired activation of adenylate cyclase by catecholamines in fat cells from hypothyroid rats can be restored by direct addition of thyroid hormone. Since catecholamine binding sites are not affected by thyroid status the defect is apparently in the coupling of the hormone-receptor complex to adenylate cyclase activation. A search is being made of steps in this coupling which might be affected by thyroid hormones with emphasis on the role of guanine nucleotides. 3. The mechanism by which growth hormone regulates, via protein synthesis, the adenylate cyclase activity of fat cells is being examined. The possible influences of thromboxanes and prostaglandins on adenylate cyclase activity is also under study. 4. The possibility that insulin and alpha adrenergic agents activate phosphatidylinositol breakdown resulting in entry of calcium is being investigated.